Implanted medical devices, such as implanted cardiac pacemakers, generally rely on a battery to provide energy for delivery of therapeutic stimulation, such as cardiac pacing. In general, when a primary battery of an implanted medical device is exhausted, the medical device must be explanted, and a new medical device implanted in its place. In some examples, medical devices have a rechargeable (secondary) battery which requires the patient to periodically recharge the battery in order for the unit to continue to function. Consequently, in order to prolong the useful life of medical devices or the time between required battery recharging, it is desirable to deliver stimulation with the lowest magnitude, e.g., electrical stimulation at the lowest current and voltage amplitude, that provides adequate therapeutic benefit. For example, it is generally desirable to deliver pacing pulses at the lowest energy that is still adequate to capture the heart.
Some implantable stimulators have used nucleonic batteries, but such stimulators are currently no longer commercially available. Other implantable stimulators have been described that generate power based on the body or heart movement, hemodynamic pressure changes, or other factors. For these devices, the size of the power generator within the implantable stimulator must be adequate for the energy consumed by the device. Therefore, even for these self-powered devices, there is an advantage to minimizing the energy used for tissue stimulation, e.g., reducing the size of the power generator and other components.
Currently available implantable cardiac pacemaker systems require the power source to be implanted distant from the heart because its size limits it from being placed in or on the heart. A cardiac lead is therefore required for directing stimulation energy from the electronics and battery package to the heart tissue. The movement of the heart and normal activity of the individual may, in some examples, subject the lead to forces that may result in lead related conditions, e.g., lead breakage or dislodgement of the electrodes from the heart tissue. Thus, in some examples, the size of the pacemaker battery also affects the efficacy of an implantable cardiac pacing system by imposing a need for a cardiac lead.
Existing techniques for prolonging the life of pacemaker power sources or decreasing the size of the power source include use of manual programming, e.g., reduction, of stimulation parameters to the minimum safe output for capture, or the use of automatic capture threshold detection algorithms to maintain pacing pulse energy at the lowest level necessary for capture. Other existing techniques are directed toward reducing the pacing pulse energy level required to capture the heart. Such techniques include use of electrode designs that concentrate current in a small area in order to increase the efficiency of stimulation.
Other electrodes incorporate a means of drug elution, such as steroids, e.g., as disclosed in U.S. Pat. No. 4,711,251 to Stokes entitled “BODY IMPLANTABLE LEAD,” which issued on Dec. 8, 1987, and is herein incorporated by reference in its entirety. With these drug eluting leads, small molecule steroids, such as dexamethasone, may diffuse from the end of the lead and into the tissue adjacent to the stimulation electrode. This localized steroid delivery may reduce chronic tissue fibrotic encapsulation which may otherwise increase the required stimulation energy over time. Steroid eluting electrodes are currently commercially available, such as those used in active and passive fixation CapSure® cardiac lead models, made commercially available Medtronic, Inc of Minneapolis, Minn.